1. Field of the Invention
This invention relates to the formation of thin films on semiconductor substrates. More particularly, this invention relates to the vaporization and transport to a deposition chamber of two or more liquid components used in the formation of the thin film on the semiconductor substrate.
2. Description of the Related Art
In the processing of thin films on a semiconductor substrate, for example, as in the formation of a silicon oxide film used in planarization, liquid source precursors or components are often used. These liquids are typically stored in source tanks and are delivered as vapors to a deposition chamber using a delivery system wherein each liquid flows through a separate line and liquid flow meter (to provide individual control of the flow rate of each reactant) and then is injected as a vapor into a common manifold. The vapors flowing in the manifold are then introduced into processing chamber connected to the manifold downstream of the points of entry of the gases and vaporized liquid source precursors into the manifold.
While the vaporous components, upon entering the processing chamber, perform satisfactorily, for example to form a thin film on a semiconductor substrate, it has been found that problems of either condensation of the previously vaporized liquid source component(s) or boiling of the still liquid component(s) in the delivery system can occur, depending upon the temperatures maintained at various points in the delivery system, including along the manifold. For example, if the temperature at a particular point along the manifold is too low (a cold spot), condensation of a previously vaporized liquid precursor source or component may occur at that point in the manifold. On the other hand, maintenance of too high a temperature in the manifold (to prevent such undesirable condensation) can result in boiling/decomposition of a liquid component in the liquid supply line of the particular liquid component upstream of its vaporization and injection into the manifold. This, in turn, can lead to instabilities in the flow rate control of that particular component due to fluctuations of the liquid flow meter as the boiling or near boiling component flows through it.
For example, in the formation of a thin film of silicon oxide on a semiconductor substrate for use as a planarization layer, the silicon oxide is usually doped with phosphorus and/or boron to enhance the flow characteristics of the silicon oxide during a subsequent planarization step. This can result in the use of a liquid silicon source precursor, such as an alkoxysilane, e.g., tetraethylorthosilicate (TEOS), a liquid phosphorus source precursor such as, for example, trimethylphosphite (TMP), triethylphosphite (TEP), or triethylphosphate (TEPO); and/or a liquid boron source precursor such as, for example, trimethylborate (TMB) or triethylborate (TEB).
These liquids are stored in separate source tanks and are delivered as vapors to a deposition chamber using a delivery system wherein the liquid sources of silicon, phosphorus, and boron flow through separate lines and liquid flow meters and then are respectively injected as vapors into a common manifold where they are usually mixed with a carrier gas. The vapors flowing in the manifold are then further mixed with a vapor source of oxygen, usually just prior to entry into a deposition chamber to avoid premature reaction, to form the doped silicon oxide film on the semiconductor substrate in the deposition chamber. Typically the reaction may be either a thermal CVD reaction or a plasma-enhanced CVD reaction. The presence of the dopants in the resulting silicon oxide film lowers the temperature at which the silicon oxide film may be subsequently reflowed to product a planarized film.
While the vaporous components, such as the reactants described above, react in a deposition chamber to form a satisfactory doped silicon oxide film useful for planarization of a structure formed on a semiconductor substrate, it has been found that problems of either condensation or boiling in the delivery system can occur. As described above, if the temperature at a particular point along the manifold is too low, condensation of a previously vaporized reactant may occur at that point in the manifold, while maintenance of too high a temperature in the manifold can result in boiling/decomposition of a liquid precursor in the liquid supply line of that reactant upstream of its vaporization and injection into the manifold, resulting in erratic flow of the liquid precursor through the liquid flow meter.
The resultant instabilities in the flow rate of the reactants, due to either problem, can interfere with the satisfactory formation of a homogeneous product such as a properly doped silicon oxide film. For example, in the above described formation of a phosphorus and/or boron-doped silicon oxide film, premature condensation can affect incorporation of one or more of the dopants into the film, as well as affecting the uniform distribution of the dopant(s) in the silicon oxide film. Additionally, each microlayer of the thin film of silicon oxide could incorporate different concentrations of the respective dopants if the vaporization rates and flow into the processing chamber are not uniform.
It would, therefore, be advantageous if a component delivery system used in the processing of thin films on semiconductor substrates, and in particular a component delivery system which utilizes liquid precursors, could be designed and operated in a manner which would reduce the temperature sensitivity of the respective components in either the vapor or liquid state.
The invention comprises a process and apparatus for the processing of thin films on semiconductor substrates using liquid precursor sources wherein the liquid precursor source or component with the highest vapor pressure is first vaporized and then introduced as a vapor into a common manifold connected to a processing chamber, with the point of introduction being spaced away from the processing chamber. A second liquid precursor source, having a vapor pressure lower than the first liquid precursor source, is then introduced in vaporized form into the manifold at a point closer to the processing chamber than the entry point of the previous vaporized liquid precursor source. This is repeated for each liquid precursor source, with each succeeding liquid precursor source having the next lower vapor pressure being introduced in vaporized form into the manifold at a point closer to the processing chamber than the previous liquid precursor source. A temperature gradient may then be maintained along the manifold with the temperature gradually increased in a direction toward the processing chamber while still mitigating premature boiling of the liquid precursors prior to vaporization, or condensation of already vaporized components.